Electrode for solid-state batteries and solid-state battery

ABSTRACT

An electrode for solid-state batteries, comprising a PTC resistor layer, and a solid-state battery comprising the electrode. The electrode may be an electrode for solid-state batteries, wherein the electrode comprises an electrode active material layer, a current collector and a PTC resistor layer which is disposed between the electrode active material layer and the current collector and which is in contact with the electrode active material layer; wherein the PTC resistor layer contains a carbon-containing electroconductive material, an insulating inorganic substance and a fluorine-containing polymer.

TECHNICAL FIELD

The disclosure relates to an electrode for solid-state batteries and asolid-state battery comprising the electrode.

BACKGROUND

In a battery used as an in-vehicle power source or as a power source fornotebook PCs and portable devices, the temperature of the whole batterymay increase due to an internal short circuit or overcharging and mayhave adverse effects on the battery itself or on a device using thebattery.

As a measure to prevent the adverse effects, a technique of using anelectrode has been attempted, the electrode comprising a positivetemperature coefficient (PTC) resistor layer which has electronconductivity at room temperature and which shows an increase inelectronic resistance value with an increase in temperature.

Patent Literature 1 discloses an all-solid-state state batterycomprising a laminate of a cathode active material layer, a solidelectrolyte layer, and an anode active material layer in this order, anda restraining member that applies a restraining pressure to the laminatein a laminated direction, wherein a PTC layer containing a conductivematerial, an insulating inorganic substance and a polymer, is disposedat least at one of a position between the cathode active material layerand a cathode current collecting layer for collecting electrons of thecathode active material layer, and a position between the anode activematerial layer and an anode current collecting layer for collectingelectrons of the anode active material layer, and the content of theinsulating inorganic substance in the PTC layer is 50 volume % or more.

Patent Literature 2 discloses an all-solid-state battery comprising: acathode layer comprising a cathode active material layer and a cathodecurrent collector; an anode layer comprising an anode active materiallayer and an anode current collector; and a solid electrolyte layerdisposed between the cathode active material layer and the anode activematerial layer, wherein the all-solid-state battery further comprises aPTC film between the cathode current collector and the cathode activematerial layer and/or between the anode current collector and the anodeactive material layer, and the PTC film contains a conductive materialand a resin.

-   Patent Literature 1: Japanese Patent Application Laid-Open (JP-A)    No. 2018-014286-   Patent Literature 2: JP-A No. 2017-130283

However, the electrode as disclosed in Patent Literature 1, theelectrode comprising the PTC resistor layer containing the insulatinginorganic substance, has a problem in that electronic resistance at theinterface between the PTC resistor layer and the electrode activematerial layer in a room temperature condition (15° C. to 30° C.) islarge. The electrode as disclosed in Patent Literature 2, the electrodecomprising the PTC resistor layer not containing the insulatinginorganic substance, has a problem in that electronic resistance isdecreased in a high temperature condition due to the effects ofconfining pressure.

SUMMARY

The disclosed embodiments were achieved in light of the abovecircumstance. An object of the disclosed embodiments is to provide anelectrode for solid-state batteries, comprising at least a PTC resistorlayer in which electronic resistance in a room temperature condition islow. Another object of the disclosed embodiments is to provide asolid-state battery comprising the electrode.

In a first embodiment, there is provided an electrode for solid-statebatteries,

wherein the electrode comprises an electrode active material layer, acurrent collector and a PTC resistor layer which is disposed between theelectrode active material layer and the current collector and which isin contact with the electrode active material layer;

wherein the PTC resistor layer contains a carbon-containingelectroconductive material, an insulating inorganic substance and afluorine-containing polymer;

wherein a hardness of an electrode active material layer-contactingsurface of the PTC resistor layer, is 0.36 GPa or less; and

wherein, when the PTC resistor layer is divided into an A layer and a Blayer in order from nearest to furthest from the electrode activematerial layer so that, at any point of the PTC resistor layer, a ratioof a thickness of the A layer to a thickness of the B layer is 1:2 in athickness direction of the PTC resistor layer,

a value obtained by dividing an atomic percentage of carbon atomscontained in the A layer by an atomic percentage of fluorine atomscontained in the A layer, is from 2.4 to 3.9.

The hardness of the electrode active material layer-contacting surfaceof the PTC resistor layer, may be from 0.22 GPa to 0.36 GPa.

The insulating inorganic substance may be a metal oxide.

The carbon-containing electroconductive material may be carbon black.

In another embodiment, there is provided a solid-state batterycomprising a cathode, an anode and an electrolyte layer disposed betweenthe cathode and the anode, wherein at least one of the cathode and theanode is the above-mentioned electrode.

For the electrode for solid-state batteries according to the disclosedembodiments, the hardness of the electrode active materiallayer-contacting surface of the PTC resistor layer, is the specificvalue or less, and the content ratio of carbon atoms with respect tofluorine atoms in the A layer that is near the electrode active materiallayer, is in the specific value range, whereby the electrode activematerial layer-contacting surface of the PTC resistor layer has betterflexibility than ever before, and excellent contact between the PTCresistor layer and the electrode active material layer is obtained,resulting in excellent followability of the PTC resistor layer to theelectrode active material layer surface. As a result, when the electrodeis used in a solid-state battery, an increase in the electronicresistance at the interface between the PTC resistor layer and theelectrode active material layer, can be suppressed, and a decrease inthe performance of the solid-state battery can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a view showing an example of the layer structure of theelectrode for solid-state batteries according to the disclosedembodiments, and it is also a schematic cross sectional view of theelectrode along the laminating direction;

FIG. 2 is a view showing an example of the layer structure of thesolid-state battery of the disclosed embodiments, and it is also aschematic cross sectional view of the solid-state battery along thelaminating direction;

FIG. 3 is a schematic view of a circuit for electronic resistancemeasurement, which includes an evaluation sample; and

FIG. 4 is a view showing a relationship between the electronicresistance of an evaluation sample of an electrode and the resistance ofa solid-state battery comprising the electrode.

DETAILED DESCRIPTION

1. Electrode for Solid-State Batteries

The electrode for solid-state batteries according to the disclosedembodiments, is an electrode for solid-state batteries,

wherein the electrode comprises an electrode active material layer, acurrent collector and a PTC resistor layer which is disposed between theelectrode active material layer and the current collector and which isin contact with the electrode active material layer;

wherein the PTC resistor layer contains a carbon-containingelectroconductive material, an insulating inorganic substance and afluorine-containing polymer;

wherein a hardness of an electrode active material layer-contactingsurface of the PTC resistor layer, is 0.36 GPa or less; and

wherein, when the PTC resistor layer is divided into an A layer and a Blayer in order from nearest to furthest from the electrode activematerial layer so that, at any point of the PTC resistor layer, a ratioof a thickness of the A layer to a thickness of the B layer is 1:2 in athickness direction of the PTC resistor layer,

a value obtained by dividing an atomic percentage of carbon atomscontained in the A layer by an atomic percentage of fluorine atomscontained in the A layer, is from 2.4 to 3.9.

It is known that if a layer containing an electroconductive material anda polymer is disposed between the electrode active material layer andthe current collector, the layer shows a PTC resistor function (a rapidincrease in electronic resistance) when the temperature of the layerexceeds the melting point of the polymer by heating. The PTC resistorfunction is exerted when the particles of the electroconductivematerial, which are in contact with each other, are separated by thermalexpansion of the polymer and result in blocking of electron conduction.In the disclosed embodiments, the layer showing such a PTC resistorfunction is referred to as “PTC resistor layer”.

In the solid-state battery comprising the PTC resistor layer, when thetemperature of the solid-state battery is increased due to overchargingor a short circuit, electron conduction between the electrode activematerial layer and the current collector is blocked, and anelectrochemical reaction is arrested. Accordingly, a further increase intemperature is suppressed and makes it possible to prevent adverseeffects on the solid-state battery itself and on a device using thesolid-state battery.

For the PTC resistor layer containing the electroconductive material andthe polymer, the polymer is deformed and fluidized when pressure isapplied to the solid-state battery, whereby the PTC resistor layercannot maintain its structure and, as a result, may fail to exert thePTC resistor function. In Patent Literature 1, for the purpose ofallowing the PTC resistor layer to maintain its structure even whenpressure is applied to the solid-state battery, the PTC resistor layerthat further contains an insulating inorganic substance, which isgenerally said to have high strength, is disclosed. It was thought thatinside the PTC resistor layer, electronic resistance is increased by theinsulating inorganic substance, thereby increasing electronic resistancein the whole electrode.

However, as a result of research, it was found that in the electrodecomprising the PTC resistor layer containing the insulating inorganicsubstance, not only the electronic resistance inside the PTC resistorlayer is high, but also the electronic resistance at the interfacebetween the PTC resistor layer and the electrode active material layer,is high. This seems to be because contact between the PTC resistor layerand the electrode active material layer at the interface therebetween,is decreased due to the presence of large amounts of the insulatinginorganic substance on the surface of the PTC resistor layer.

For the electrode for solid-state batteries according to the disclosedembodiments, the hardness of the electrode active materiallayer-contacting surface of the PTC resistor layer, is the specificvalue or less, and the content ratio of carbon atoms with respect tofluorine atoms in the A layer that is near the electrode active materiallayer, is in the specific value range. As a result, when the electrodeis used in a solid-state battery, a decrease in the performance of thesolid-state battery can be suppressed.

The electrode for solid-state batteries according to the disclosedembodiments comprises an electrode active material layer, a currentcollector and a PTC resistor layer.

FIG. 1 is a view showing an example of the layer structure of theelectrode for solid-state batteries according to the disclosedembodiments, and it is also a schematic cross sectional view of theelectrode along the laminating direction. As shown in FIG. 1, anelectrode 10 for solid-state batteries according to the disclosedembodiments, comprises an electrode active material layer 2, a currentcollector 3, and a PTC resistor layer 1 disposed between the electrodeactive material layer 2 and the current collector 3.

As shown in FIG. 1, the PTC resistor layer 1 is in contact with theelectrode active material layer 2. Also, as shown in FIG. 1, the PTCresistor layer 1 may be in contact with the current collector 3. Adifferent layer (not shown in FIG. 1) may be present between the PTCresistor layer 1 and current collector 3 of the electrode 10 forsolid-state batteries.

The PTC resistor layer 1 includes an A layer 1 a, which is present nearthe electrode active material layer 2, and a B layer 1 b, which ispresent near the current collector 3. As will be described below, thecontent ratio of carbon atoms with respect to fluorine atoms in the Alayer 1 a, is in the specific range.

Hereinafter, these layers of the electrode for solid-state batterieswill be described in detail.

(1) PTC Resistor Layer

The PTC resistor layer is a layer which contains a carbon-containingelectroconductive material, an insulating inorganic substance and afluorine-containing polymer, which is disposed between the electrodeactive material layer and the current collector, and which is in contactwith the electrode active material layer.

The carbon-containing electroconductive material contained in the PTCresistor layer is not particularly limited, as long as it containscarbon atoms and it has electroconductivity. As the carbon-containingelectroconductive material, examples include, but are not limited to,carbon black, activated carbon, carbon fiber (e.g., carbon nanotube,carbon nanofiber) and graphite. The carbon-containing electroconductivematerial contained in the PTC resistor layer may be carbon black. Thecarbon-containing electroconductive material may be in a particulateform. As the particulate form, examples include, but are not limited to,a fibrous form.

The volume ratio of the carbon-containing electroconductive material inthe PTC resistor layer is not particularly limited. When the totalvolume of the carbon-containing electroconductive material, theinsulating inorganic substance and the fluorine-containing polymer isdetermined as 100 volume %, the volume ratio of the carbon-containingelectroconductive material in the PTC resistor layer may be from 7volume % to 50 volume %, or it may be from 7 volume % to 10 volume %.

The insulating inorganic substance contained in the PTC resistor layerfunctions to suppress deformation and fluidization of the PTC resistorlayer in the electrode for solid-state batteries, both of which are dueto high temperature and pressure.

The insulating inorganic substance is not particularly limited, as longas it is a material that has a higher melting point than thebelow-described fluorine-containing polymer. As the insulating inorganicsubstance, examples include, but are not limited to, a metal oxide and ametal nitride. As the metal oxide, examples include, but are not limitedto, alumina, zirconia and silica. As the metal nitride, examplesinclude, but are not limited to, a silicon nitride. Also, as theinsulating inorganic substance, examples include, but are not limitedto, a ceramic material. The insulating inorganic substance may be ametal oxide.

In general, the insulating inorganic substance is in a particulate form.The insulating inorganic substance may be primary particles or secondaryparticles.

The average particle diameter (D₅₀) of the insulating inorganicsubstance may be from 0.2 μm to 5 μm, or it may be from 0.4 μm to 2 μm,for example. The particle size distribution of the insulating inorganicsubstance particles is not particularly limited. The particle sizedistribution of the particles may be a normal distribution when it isrepresented by a frequency distribution.

The volume ratio of the insulating inorganic substance in the PTCresistor layer is not particularly limited. When the total volume of thecarbon-containing electroconductive material, the insulating inorganicsubstance and the fluorine-containing polymer is determined as 100volume %, the volume ratio of the insulating inorganic substance in thePTC resistor layer may be from 40 volume % to 85 volume %, or it may befrom 50 volume % to 60 volume %.

When the volume ratio of the insulating inorganic substance in the PTCresistor layer is too small, it may be difficult to sufficientlysuppress the deformation and fluidization of the PTC resistor layer,both of which are due to heating and pressure. On the other hand, whenthe volume ratio of the insulating inorganic substance in the PTCresistor layer is too large, the volume ratio of the fluorine-containingpolymer is relatively small. As a result, the effect of separating theparticles of the carbon-containing electroconductive material by thefluorine-containing polymer may be insufficiently exerted, and anincrease in electronic resistance may be insufficient. Also when thevolume ratio of the insulating inorganic substance in the PTC resistorlayer is too large, electroconductive paths, which are formed by thecarbon-containing electroconductive material, may be blocked by theinsulating inorganic substance, and the electron conductivity of the PTCresistor layer during normal use may decrease. In the disclosedembodiments, the electron conductivity of the PTC resistor layer meansthe property of conducting electrons through the PTC resistor layer, andit is strictly different from the electroconductivity of the PTCresistor layer (the property of conducting electricity through the PTCresistor layer).

The fluorine-containing polymer contained in the PTC resistor layer isnot particularly limited, as long as it is a polymer which containsfluorine and which expands when its temperature exceeds its meltingpoint by heating. As the fluorine-containing polymer, examples include,but are not limited to, polyvinylidene fluoride (PVDF),polyfluoroethylene (PTFE), perfluoroalkoxy alkane (PFA), perfluoroethylene-propylene copolymer (FEP) and polychlorotrifluoroethylene(PCTFE). These fluorine-containing polymers may be used alone or incombination of two or more kinds.

From the viewpoint of melting point and ease of processing, thefluorine-containing polymer may be polyvinylidene fluoride orpolyfluoroethylene. The fluorine-containing polymer may bepolyvinylidene fluoride.

The volume ratio of the fluorine-containing polymer in the PTC resistorlayer is not particularly limited. When the total volume of thecarbon-containing electroconductive material, the insulating inorganicsubstance and the fluorine-containing polymer is determined as 100volume %, the volume ratio of the fluorine-containing polymer in the PTCresistor layer may be from 8 volume % to 60 volume %, or it may be from8 volume % to 45 volume %.

The thickness of the PTC resistor layer is not particularly limited. Itmay be from about 1 μm to about 30 μm.

As described above, the electrode comprising the PTC resistor layercontaining the insulating inorganic substance, has a problem in that theelectronic resistance at the interface between the PTC resistor layerand the electrode active material layer in a room temperature condition(15° C. to 30° C.) is large. As just described, when the insulatinginorganic substance is used to increase the electronic resistance in ahigh temperature condition, the electronic resistance in a roomtemperature condition also increases. However, in the case of decreasingonly the content ratio of the insulating inorganic substance, theelectronic resistance in a high temperature condition decreases, so thatthe PTC resistor layer cannot sufficiently function.

Accordingly, to decrease the electronic resistance in a room temperaturecondition while keeping the high electronic resistance in a hightemperature condition, it is needed to enhance the flexibility, contactand followability of the PTC resistor layer with respect to theelectrode active material layer. As just described, by enhancing thephysical properties of the PTC resistor layer, the number of electroniccontact points disposed at the interface between the PTC resistor layerand the electrode active material layer, can be increased more than everbefore. The electronic contact points mean contact points between thePTC resistor layer and the electrode active material layer, at which atleast electron conduction is possible. As a result, in a roomtemperature condition, the electronic resistance at the interfacebetween the PTC resistor layer and the electrode active material layercan be suppressed, and a decrease in the performance of the solid-statebattery comprising the electrode for solid-state batteries according tothe disclosed embodiments, can be suppressed. On the other hand, sinceit is not needed to decrease the amount of the insulating inorganicsubstance in the PTC resistor layer, there is no decrease in electronicresistance in a high temperature condition, and the function of stoppingcharging/discharging of the solid-state battery can be exerted at thetime of occurrence of a failure, confining the solid-state battery, etc.

In the disclosed embodiments, the hardness of the electrode activematerial layer-contacting surface of the PTC resistor layer, is used asthe indicator of the flexibility, contact and followability of the PTCresistor layer with respect to the electrode active material layer. Thesmaller the hardness, the better the flexibility of the electrode activematerial layer-contacting surface of the PTC resistor layer.Accordingly, excellent contact between the PTC resistor layer and theelectrode active material layer is obtained, resulting in excellentfollowability of the PTC resistor layer to the electrode active materiallayer surface and more electronic contact points between the PTCresistor layer and the electrode active material layer than ever before.As a result, an increase in the electronic resistance between the PTCresistor layer and the electrode active material layer in a roomtemperature condition, can be suppressed, and a decrease in theperformance of the solid-state battery can be suppressed.

In the disclosed embodiments, the hardness of the electrode activematerial layer-contacting surface of the PTC resistor layer, is hardnessmeasured by the nanoindentation test according to ISO14577:2015 (Part.1: Test method).

A sample is used in the nanoindentation test, and it may be the PTCresistor layer itself or may be a laminate in which the PTC resistorlayer appears on at least one surface of the laminate. As the laminate,examples include, but are not limited to, a laminate of the PTC resistorlayer and the current collector, a laminate of the PTC resistor layerand the electrode active material layer, and a laminate of the PTCresistor layer and a substrate. The substrate means a material whichserves as a support for forming the PTC resistor layer and which isother than the current collector and the electrode active materiallayer. Typically, it means a substrate for transferring the PTC resistorlayer.

The laminate of the PTC resistor layer and the current collector may beone that is obtained from the electrode or the solid-state batterycomprising the electrode, or it may be a laminate A used for productionof evaluation samples described below.

The laminate of the PTC resistor layer and the electrode active materiallayer may be one that is obtained from the electrode or the solid-statebattery comprising the electrode, or it may be a laminate B used for theproduction of evaluation samples described below.

The method for measuring the hardness of the electrode active materiallayer-contacting surface of the PTC resistor layer, is as follows.

For the above-described sample, the hardness of the surface on which thePTC resistor layer appears, is measured by use of a nanoindenter(product name: G200, manufactured by: MTS Systems Corporation) and inaccordance with ISO 14577:2015 (Part. 1: Test method). Morespecifically, 15 points at an indentation depth of from 400 nm to 600 nmare measured for hardness (GPa).

The average of the hardness values thus obtained is determined as thehardness of the electrode active material layer-contacting surface ofthe PTC resistor layer.

The hardness of the electrode active material layer-contacting surfaceof the PTC resistor layer, is 0.36 GPa or less. If the hardness is morethan 0.36 GPa, the flexibility, contact and followability of the PTCresistor layer with respect to the electrode active material layer, areall insufficient and the number of the electronic contact points betweenthe layers are small. Accordingly, even if the electrode with such ahardness is used in the solid-state battery, it is difficult to suppressa decrease in the performance of the solid-state battery.

The hardness of the electrode active material layer-contacting surfaceof the PTC resistor layer, may be from 0.22 GPa to 0.36 GPa, may be from0.23 GPa to 0.35 GPa, or may be from 0.24 GPa to 0.34 GPa. In general,it is often technically difficult to control the hardness of theelectrode active material layer-contacting surface of the PTC resistorlayer to less than 0.22 GPa, depending on the type and content ratio ofmaterials contained in the PTC resistor layer.

The hardness of the electrode active material layer-contacting surfaceof the PTC resistor layer, can be controlled to 0.36 GPa or less by, forexample, like the below-described production method, forming the PTCresistor layer by applying a second slurry to the surface of a firstcoating layer. In this case, the content of the insulating inorganicsubstance in the second slurry may be zero, or the volume ratio (on thebasis of solid content) of the insulating inorganic substance containedin the second slurry may be smaller than the volume ratio of theinsulating inorganic substance in the first coating layer.

Also, the hardness can be achieved by a pressing method such as rollpressing (which will be described below), cold isostatic pressing (CIP)and hot isostatic pressing (HIP). However, applying the second slurry tothe first coating layer surface is more effective in controlling thehardness to 0.36 GPa or less, than pressing the laminate including thePTC resistor layer.

Controlling the amount of the carbon-containing electroconductivematerial contained in the second slurry, controlling the thickness ofthe applied second slurry, etc., are also effective in reducing thethickness further.

The method for measuring the hardness of the electrode active materiallayer-contacting surface of the PTC resistor layer included in thethus-obtained solid-state battery, is not particularly limited. Forexample, the hardness can be measured by the following method. First,the below-described solid-state battery is disassembled in an inertatmosphere (e.g., Ar gas atmosphere) and the PTC resistor layer is takenout from the disassembled solid-state battery.

Then, for a surface of the PTC resistor layer, which was in contact withthe electrode active material layer before disassembling the solid-statebattery, the hardness is measured by use of the nanoindenter (productname: G200, manufactured by: MTS Systems Corporation) and in accordancewith ISO 14577:2015 (Part. 1: Test method). More specifically, 15 pointsat an indentation depth of from 400 nm to 600 nm are measured forhardness (GPa).

The average of the hardness values thus obtained is determined as thehardness of the electrode active material layer-contacting surface ofthe PTC resistor layer.

In the disclosed embodiments, the PTC resistor layer is divided into anA layer and a B layer in order from nearest to furthest from theelectrode active material layer so that, at any point of the PTCresistor layer, the ratio of the thickness of the A layer to thethickness of the B layer is 1:2 in the thickness direction of the PTCresistor layer. The thickness direction of the PTC resistor layer meansa direction vertical to a direction in which the PTC resistor layerextends. In other words, it means a direction parallel to the laminatingdirection of the laminate of the electrode active material layer, thePTC resistor layer and the current collector.

The A layer is a layer starting from the interface between the PTCresistor layer and the electrode active material layer and occupyingone-third of the thickness of the PTC resistor layer. Meanwhile, the Blayer is a layer starting from the interface between the PTC resistorlayer and a layer disposed on the opposite side of the electrode activematerial layer (e.g., from the interface between the PTC resistor layerand a current collector layer) and occupying two-thirds of the thicknessof the PTC resistor layer. A main feature of the disclosed embodimentsis that the content ratio of the carbon atoms with respect to thefluorine atoms in the A layer is in the specific value range.

In the disclosed embodiments, the A layer and the B layer are defined sothat, at any point of the PTC resistor layer, the ratio of the thicknessof the A layer to the thickness of the B layer is 1:2 in the thicknessdirection of the PTC resistor layer. Actually, since the A layer and theB layer are combined to form the PTC resistor layer in many cases, it isdifficult to absolutely separate them. The above definition is merely adefinition that is conveniently provided to define the content ratio ofthe carbon atoms with respect to the fluorine atoms in a part of the PTCresistor layer, which is on the side near the electrode active materiallayer.

The value obtained by dividing the atomic percentage (atomic %) of thecarbon atoms (C) contained in the A layer by the atomic percentage(atomic %) of the fluorine atoms (F) contained in the A layer(hereinafter the value may be referred to as C/F value) is generallyfrom 2.4 to 3.9. The C/F value may be from 2.5 to 3.8, or it may be from2.6 to 3.7.

Since the C/F value of the A layer, which is on the side near theelectrode active material layer, of the PTC resistor layer is in theabove range, the thermal stability of the whole PTC resistor layer isincreased, and the electronic resistance value of the whole PTC resistorlayer in a high temperature condition is more increased. As a result, inthe case of using the electrode for solid-state batteries according tothe disclosed embodiments in a solid-state battery, progression ofbattery reaction can be suppressed when the battery temperature isincreased due to an internal short circuit, etc.

When the C/F value is smaller than 2.4, the carbon amount in the A layeris too small. As a result, a sufficient number of electroconductivepaths are not ensured at the interface between the PTC resistor layerand the electrode active material layer. On the other hand, when the C/Fvalue is larger than 3.9, the carbon amount in the A layer is too large.As a result, the thermal stability of the whole PTC resistor layer isdecreased.

An indicator of the thermal stability may be the electronic resistancein a high temperature condition (e.g., 250° C.), for example. As theelectronic resistance in a high temperature condition increases, theelectrode for solid-state batteries obtains better thermal stability.

In the disclosed embodiments, the method for calculating the C/F valueis as follows.

First, the electrode for solid-state batteries is subjected to crosssection polishing by a cross section polisher (CP), thereby obtaining across section. The cross section of the PTC resistor layer is observedwith a field emission-scanning electron microscope (FE-SEM), therebyobtaining a SEM image.

Next, on the SEM image, the PTC resistor layer is divided into the Alayer and the B layer in order from nearest to furthest from theelectrode active material layer so that, at any point of the PTCresistor layer, the ratio of the thickness of the A layer to thethickness of the B layer is 1:2 in the thickness direction of the PTCresistor layer. Accordingly, the cross-sectional area of the A layer isone-third of the cross-sectional area of the whole PTC resistor layer,and the cross-sectional area of the B layer is two-thirds of thecross-sectional area of the whole PTC resistor layer. There is apossibility that the A layer is not absolutely the same as a secondcoating layer used in a production method described below (a layer onthe side near the cathode active material layer), and there is apossibility that the B layer is not absolutely the same as a firstcoating layer used in the production method described below (a layer onthe side near the current collector).

Next, a cross section of the A layer of the PTC resistor layer issubjected to elemental analysis by flat quad-energy dispersive X-rayspectroscopy (FQ-EDX). The cross section of the A layer of the PTCresistor layer is a cross section of the A layer defined from theabove-described SEM image.

Next, from the results of the elemental analysis, the amount (atomic %)of C contained in the cross section of the A layer and the amount of F(atomic %) contained therein are calculated. The C amount (atomic %) isdivided by the F amount (atomic %), and the resulting value is used asthe C/F value of the electrode for solid-state batteries.

In this evaluation method, in place of the electrode for solid-statebatteries, a solid-state battery comprising the electrode forsolid-state batteries may be used, or an evaluation sample describedbelow may be used.

The contact area between the PTC resistor layer and the electrode activematerial layer is not particularly limited. As long as 50% or more, 70%or more, or 99% or more of the area of the electrode active materiallayer is in contact with the PTC resistor layer, the effect ofsuppressing the electronic resistance at the interface between the PTCresistor layer and the electrode active material layer, is sufficientlyexerted.

The contact area between the PTC resistor layer and the electrode activematerial layer means an area where the PTC resistor layer and theelectrode active material layer are apparently in contact with eachother, regardless of the presence or absence of the electronic contactpoints between the layers.

(2) Electrode Active Material Layer

The electrode active material layer is not particularly limited, as longas it contains at least an electrode active material. As needed, it maycontain a binder, an electroconductive material, and a solidelectrolyte.

In the case of using the electrode for solid-state batteries accordingto the disclosed embodiments as the cathode, the electrode activematerial is not particularly limited, as long as it is an electrodeactive material that is generally used as a cathode active material. Forexample, when transferred ions are lithium ions, as the cathode activematerial, examples include, but are not limited to, a compound having alayered structure (such as LiCoO₂ and LiNiO₂), a compound having aspinel-type structure (such as LiMn₂O₄), and a compound having anolivine-type structure (such as LiFePo₄).

In the case of using the electrode for solid-state batteries accordingto the disclosed embodiments as the anode, the electrode active materialis not particularly limited, as long as it is an electrode activematerial that is generally used as an anode active material. Forexample, when the transferred ions are lithium ions, as the anode activematerial, examples include, but are not limited to, a carbonaceousmaterial, a lithium alloy, an oxide and a nitride.

The binder is not particularly limited, as long as it is chemically andelectrically stable. As the binder, examples include, but are notlimited to, a fluorine-containing binder such as polyvinylidene fluoride(PVDF) and polytetrafluoroethylene (PTFE).

The electroconductive material is not particularly limited, as long asit has electroconductivity. As the electroconductive material, examplesinclude, but are not limited to, carbonaceous materials such as carbonblack, activated carbon, carbon fiber (e.g., carbon nanotube, carbonnanofiber) and graphite.

The material for the solid electrolyte is not particularly limited, aslong as it has ion conductivity. As the material, examples include, butare not limited to, inorganic materials such as a sulfide material andan oxide material. As the sulfide material, examples include, but arenot limited to, Li₂S—SiS₂, LiI—Li₂S—SiS₂, LiI—Li₂S—P₂S₅,LiI—Li₂O—Li₂S—P₂S₅, LiI—Li₂S—P₂O₅, LiI—Li₃PO₄—P₂S₅, Li₂S—P₂S₅, Li₃PS₄,LiI—LiBr—Li₂S—P₂S₅ and Li₂S—P₂S₅—GeS₂.

(3) Current Collector

The material for the current collector is not particularly limited, aslong as it has electron conductivity. As the material for the currentcollector, examples include, but are not limited to, Al, Cu, Ni, SUS andFe. In the case of using the electrode for solid-state batteriesaccording to the disclosed embodiments as the cathode, Al may be used asthe material for the current collector. In the case of using theelectrode for solid-state batteries according to the disclosedembodiments as the anode, Cu may be used as the material for the currentcollector.

(4) Properties of Electrode for Solid-State Batteries

When the solid-state battery is in normal use, the value of theelectronic resistance of the electrode for solid-state batteries in aroom temperature condition (15° C. to 30° C.) may be small. From theviewpoint of exerting the battery reaction stopping function at hightemperature, the value of the electronic resistance of the electrode forsolid-state batteries in a high temperature condition may be large.

The ratio between the electronic resistance value of the electrode forsolid-state batteries in a high temperature condition and the electronicresistance value of the electrode for solid-state batteries in a roomtemperature condition, may be 1.5 or more, may be 2 or more, or may be 5or more. When the ratio is too small, there is a possibility that boththe excellent battery properties in normal use and the feature ofstopping the battery at the time of occurrence of a failure, confiningthe solid-state battery, etc., are not obtained. Also when the ratio istoo small, there is a possibility that the electrode for solid-statebatteries is poor in thermal stability.

The ratio between the electronic resistance value of the electrode forsolid-state batteries in a high temperature condition and the electronicresistance value of the electrode for solid-state batteries in a roomtemperature condition, may be 20,000 or less.

(5) Method for Producing the Electrode for Solid-State Batteries

The method for producing the electrode for solid-state batteries is notparticularly limited, as long as the above-described electrode forsolid-state batteries can be obtained by the method. Hereinafter, twoembodiments of the method for producing the electrode for solid-statebatteries will be described. The method for producing the electrode forsolid-state batteries according to the disclosed embodiments, is notlimited to the two embodiments.

A. First Embodiment

The first embodiment of the method for producing the electrode forsolid-state batteries comprises (a) forming the first coating layer onone surface of the current collector, (b) forming the PTC resistor layerby applying the second slurry to the surface of the first coating layer,and (c) laminating the electrode active material layer on the PTCresistor layer.

(a) Forming the First Coating Layer on One Surface of the CurrentCollector

This is a step of forming the first coating layer by applying the firstslurry to one surface of the current collector and drying the appliedfirst slurry.

The first slurry contains a carbon-containing electroconductivematerial, an insulating inorganic substance and a fluorine-containingpolymer. Details of the materials are as described above. The contentratio of the carbon-containing electroconductive material, theinsulating inorganic substance and the fluorine-containing polymer inthe first slurry and in the below-described second slurry, may beappropriately determined so as to correspond to the volume ratio anddistribution of the carbon-containing electroconductive material, theinsulating inorganic substance and the fluorine-containing polymer inthe PTC resistor layer of the electrode for solid-state batteries.

For the content ratio of the materials in the first slurry, thecarbon-containing electroconductive material, the fluorine-containingpolymer and the insulating inorganic substance may be at a volume ratioof 10:30:60, for example.

The first slurry may contain a non-aqueous solvent for dissolving ordispersing the carbon-containing electroconductive material, theinsulating inorganic substance and the fluorine-containing polymer. Thetype of the non-aqueous solvent is not particularly limited. As thenon-aqueous solvent, examples include, but are not limited to,N-methylpyrrolidone, acetone, methyl ethyl ketone and dimethylacetamide.From the viewpoint of safety such as high flash point, small influenceon human body and so on, the non-aqueous solvent may beN-methylpyrrolidone.

The content ratio of the non-aqueous solvent in the first slurry is notparticularly limited. When the total volume of the first slurry isdetermined as 100 volume %, the non-aqueous solvent may be from 80volume % to 93 volume %, or it may be from 82 volume % to 90 volume %.

The method for forming the first coating layer is not particularlylimited. In general, the first slurry in which the carbon-containingelectroconductive material, the insulating inorganic substance and thefluorine-containing polymer are dispersed in the non-aqueous solvent, isapplied onto the current collector, and the applied slurry is dried. Touniformly form the first coating layer, the solid content concentrationof the first slurry containing the carbon-containing electroconductivematerial, the insulating inorganic substance and the fluorine-containingpolymer, may be from 13 mass % to 40 mass %.

The thickness of the first coating layer is not particularly limited. Itmay be from about 1 μm to about 30 μm.

The condition for drying the first slurry is not particularly limited.For example, it may be a temperature condition in which the non-aqueoussolvent can be distilled away.

(b) Forming the PTC Resistor Layer by Applying the Second Slurry to theSurface of the First Coating Layer

This is a step of forming the PTC resistor layer by applying the secondslurry to the surface of the first coating layer on the currentcollector and drying the applied second slurry. The PTC resistor layeris a layer comprising the solid content of the second slurry and thefirst coating layer.

The second slurry contains a carbon-containing electroconductivematerial and a fluorine-containing polymer. The second slurry mayfurther contain an insulating inorganic substance. When the insulatinginorganic substance is not contained in the second slurry, contactbetween the PTC resistor layer and the electrode active material layercan be better compared to the case where the second slurry contains theinsulating inorganic substance.

For the content ratio of the materials in the second slurry, in the casewhere the insulating inorganic substance is not contained in the secondslurry, the electroconductive material and the polymer may be at avolume ratio of from 85:15 to 20:80, for example.

The content ratio of the non-aqueous solvent in the second slurry is notparticularly limited. When the total volume of the second slurry isdetermined as 100 volume %, the non-aqueous solvent may be from 75volume % to 95 volume %, or it may be from 85 volume % to 90 volume %.

The method for applying and drying the second slurry is not particularlylimited. In general, the second slurry in which the carbon-containingelectroconductive material and the fluorine-containing polymer aredispersed in the non-aqueous solvent, is applied onto the currentcollector, and the applied slurry is dried. To uniformly apply thesecond slurry, the solid content concentration of the second slurrycontaining at least the carbon-containing electroconductive material andthe fluorine-containing polymer, may be from 13 mass % to 35 mass %.

The thickness of the layer corresponding to the part formed by applyingand drying the second slurry (hereinafter, the layer may be referred toas “second coating layer”) is not particularly limited. The thicknessmay be from about 1 μm to about 10 μm, or it may be from about 2 μm toabout 6 μm. The thickness of the second coating layer is obtained from,for example, a difference between the thickness of the laminate beforethe second coating layer is formed and the thickness of the laminateafter the second coating layer is formed.

In general, after the second slurry is applied and dried, the firstcoating layer and the solid content of the second slurry are combined toform the PTC resistor layer.

Before laminating the electrode active material layer on the PTCresistor layer, the laminate of the current collector and the PTCresistor layer may be pressed. The laminate may be pressed by rollpressing, cold isostatic pressing (CIP), hot isostatic pressing (HIP),etc. When the applied pressing pressure is too high, the PTC resistorlayer may be cracked. For example, in the case of roll pressing, thepressing pressure may be a line pressure of from 5.6 kN/cm to 14.2kN/cm.

By pressing the laminate of the current collector and the PTC resistorlayer, the hardness of the electrode active material layer-contactingsurface of the PTC resistor layer can be controlled to 0.25 GPa or less(see Examples 3 and 4).

(c) Laminating the Electrode Active Material Layer on the PTC ResistorLayer

By laminating the electrode active material layer on the PTC resistorlayer, a laminate of the electrode active material layer, the PTCresistor layer and the current collector is produced. Details of thematerials that can be used to form the electrode active material layer(an electrode active material, a binder and a solid electrolyte) are asdescribed above.

As the method for forming the electrode active material layer, a knowntechnique may be used. For example, the electrode active material layercan be formed as follows: a mixture of raw materials for the electrodeactive material layer is stirred well; the raw material mixture isapplied onto a substrate or onto the PTC resistor layer; and the appliedraw material mixture is appropriately dried, thereby forming theelectrode active material layer.

In the case of forming the electrode active material layer on asubstrate, roll pressing in a high temperature condition (hot rollpressing) may be used. By hot roll pressing, the electrode activematerial layer thus obtained can be more densified. In the case offorming the electrode active material layer on the PTC resistor layer,if the heating temperature of the hot roll pressing is too high, thereis a possibility that the polymer in the PTC resistor layer is thermallyexpanded. Accordingly, it is needed to determine the upper limittemperature of the hot roll pressing, depending on the properties of thepolymer, the composition of the PTC resistor layer, etc. In general, thehot roll pressing may be carried out at a temperature less than themelting point of the polymer.

B. Second Embodiment

The second embodiment of the method for producing the electrode forsolid-state batteries, comprises (a) forming the first coating layer onone surface of the current collector, (b′) forming the second coatinglayer on one surface of the electrode active material layer, and (c′)producing a laminate of the current collector, the PTC resistor layerand the electrode active material layer.

Of them, (a) is the same as the first embodiment described above.Hereinafter, (b′) and (c′) will be described.

(b′) Forming the Second Coating Layer on One Surface of the ElectrodeActive Material Layer

This is a step of forming the second coating layer on the electrodeactive material layer by applying the second slurry to one surface of asubstrate, drying the applied second slurry to form the second coatinglayer, and then transferring the second coating layer from the substrateto the electrode active material layer.

In the first embodiment, as described above in (b), the second coatinglayer is formed on the first coating layer. In this step of the secondembodiment, the second coating layer is formed on the electrode activematerial layer. As just described, the two embodiments differ in themember on which the second coating layer is formed.

Transferring the second coating layer from the substrate to theelectrode active material layer, is advantageous in that the solventused in the second slurry has no influence on the electrode activematerial layer.

The second slurry and the thus-obtained second coating layer are thesame as those of the first embodiment.

The substrate used to form the second coating layer is not particularlylimited. For example, Al, PET, Cu, SUS or the like may be used.

(c′) Producing the Laminate of the Electrode Active Material Layer, thePTC Resistor Layer and the Current Collector

In this step, the current collector and the electrode active materiallayer are laminated so that the first coating layer of the currentcollector and the second coating layer of the electrode active materiallayer are in contact with each other, whereby the first coating layerand the second coating layer are combined to form the PTC resistorlayer. As a result, the laminate of the electrode active material layer,the PTC resistor layer and the current collector is formed.

(6) Measurement of the Electronic Resistance of the Electrode forSolid-State Batteries

An evaluation item of the electrode for solid-state batteries iselectronic resistance measurement. For the electronic resistancemeasurement, a solid-state battery comprising the electrode forsolid-state batteries or an evaluation sample comprising the electrodefor solid-state batteries, is used.

Hereinafter, the evaluation sample will be described. FIG. 3 is aschematic cross-sectional view of an evaluation sample including theelectrode for solid-state batteries according to the disclosedembodiments. An electrode 10 for solid-state batteries shown in FIG. 3corresponds to the electrode 10 for solid-state batteries shown in FIG.1 and to an electrode 10 for solid-state batteries shown in FIG. 2. InFIG. 3, the A layer and the B layer are not shown and are omitted forclarity.

As shown in FIG. 3, the layer structure of an evaluation sample 50 is asfollows: current collector 3/PTC resistor layer 1/cathode activematerial layer 2/current collector 3′/cathode active material layer2/PTC resistor layer 1/current collector 3. As is clear from FIG. 3, theevaluation sample 50 is formed by disposing the current collector 3′between the cathode active material layers 2 of the two electrodes 10for solid-state batteries, the layers facing each other.

An example of the method for producing the evaluation sample is asfollows. First, two laminates of the PTC resistor layer and the currentcollector (hereinafter, each laminate may be referred to as laminate A)and two laminates of the cathode active material layer and the currentcollector (hereinafter, each laminate may be referred to as laminate B)were produced. Next, the two laminates B are laminated so that thecathode active material layer of one laminate B and the currentcollector of the other laminate B are in contact with each other. From alaminate thus obtained, the current collector disposed outside is peeledoff, thereby producing a laminate having the following layer structure:cathode active material layer/current collector/cathode active materiallayer (hereinafter, the laminate may be referred to as laminate C). Thelaminate C corresponds to the central part (cathode active materiallayer 2/current collector 3′/cathode active material layer 2) of theevaluation sample 50 shown in FIG. 3. Finally, the two laminates A arelaminated on both surfaces of the laminate C so that the cathode activematerial layers are in contact with the PTC resistor layers, therebyproducing the evaluation sample 50 shown in FIG. 3.

FIG. 3 is a schematic view of a circuit for electronic resistancemeasurement, which includes an evaluation sample. As shown in FIG. 3, atester 40 is connected to the evaluation sample 50, thereby producing acircuit 200 for electronic resistance measurement. The electronicresistance of the evaluation sample 50 in a room temperature condition(e.g., 25° C.) and the electronic resistance thereof in a hightemperature condition (e.g., 250° C.) can be measured by use of thecircuit 200 for electronic resistance measurement.

In place of the evaluation sample 50 shown in FIG. 3, a solid-statebattery described below may be used for the electronic resistancemeasurement.

FIG. 4 is a view showing a relationship between the electronicresistance of an evaluation sample including a PTC resistor layer andthe resistance of a solid-state battery comprising an electrodeincluding the PTC resistor layer. FIG. is a graph with the resistance(Ω·cm²) of the solid-state battery on the vertical axis and theelectronic resistance (Ω·cm²) of the evaluation sample on the horizontalaxis.

As is clear from FIG. 4, the resistance of the solid-state batteryincreases as the electronic resistance of the evaluation sampleincreases. As just described, since the electronic resistance of theevaluation sample and the resistance of the solid-state battery arehighly correlated with each other, the result of the electronicresistance measurement of the evaluation sample can be said to be a testresult reflecting the performance of the solid-state battery itself.

2. Solid-State Battery

The solid-state battery of the disclosed embodiments is a solid-statebattery comprising a cathode, an anode and an electrolyte layer disposedbetween the cathode and the anode, wherein at least one of the cathodeand the anode is the above-mentioned electrode for solid-statebatteries.

In the disclosed embodiments, the solid-state battery means a batterycontaining a solid electrolyte. Accordingly, as long as the solid-statebattery of the disclosed embodiments contains a solid electrolyte, thesolid-state battery may be fully composed of a solid component, or itmay contain both a solid component and a liquid component.

FIG. 2 is a view showing an example of the layer structure of thesolid-state battery of the disclosed embodiments, and it is also aschematic cross sectional view of the solid-state battery along thelaminating direction. As shown in FIG. 2, a solid-state battery 100comprises the electrode 10 for solid-state batteries, an oppositeelectrode 30, and an electrolyte layer 20 disposed between the electrode10 for solid-state batteries and the opposite electrode 30.

The electrode 10 for solid-state batteries corresponds to theabove-described electrode for solid-state batteries according to thedisclosed embodiments. The opposite electrode is an electrode facing theelectrode 10 for solid-state batteries. The electrode 10 for solid-statebatteries and the opposite electrode 30 may be the cathode and theanode, respectively; the electrode 10 for solid-state batteries and theopposite electrode 30 may be the anode and the cathode, respectively;or, unlike FIG. 2, each of the cathode and the anode may be theelectrode for solid-state batteries according to the disclosedembodiments.

The electrode 10 for solid-state batteries is as described above. Theopposite electrode 30, that is, a cathode or anode that is generallyused in the solid-state battery, may be selected from known techniques.Especially, the cathode active material layer and cathode currentcollector which can be used in the cathode, and the anode activematerial layer and anode current collector which can be used in theanode, may be appropriately selected from the above-described materialsused in the disclosed embodiments.

The electrolyte layer 20 is not particularly limited, as long as it is alayer having ion conductivity. The electrolyte layer 20 may be a layercomposed of a solid electrolyte only, or it may be a layer containingboth a solid electrolyte and a liquid electrolyte.

As the electrolyte layer composed of the solid electrolyte only,examples include, but are not limited to, a polymer solid electrolytelayer, an oxide solid electrolyte layer and a sulfide solid electrolytelayer.

As the electrolyte layer containing both the solid electrolyte and theliquid electrolyte, examples include, but are not limited to, a poroussolid electrolyte layer impregnated with an aqueous or non-aqueouselectrolyte solution.

The form of the solid-state battery of the disclosed embodiments is notparticularly limited. As the form of the solid-state battery, examplesinclude, but are not limited to, common forms such as a coin form, aflat plate form and a cylindrical form.

The solid-state battery of the disclosed embodiments may be a singlecell as shown in FIG. 2, or it may be an assembly of the single cells.As the cell assembly, examples include, but are not limited to, a cellstack composed of a stack of single cells in a flat plate form.

As described above, in the pressure-applied condition, the electrode forsolid-state batteries according to the disclosed embodiments exerts theexcellent effect of suppressing a decrease in solid-state batteryperformance. Accordingly, the electrode for solid-state batteriesaccording to the disclosed embodiments exerts the excellent effect evenwhen an unintentional pressure is applied (such as occurrence of afailure in the solid-state battery due to an internal short circuit,overcharging, etc.) or even when an intentional pressure is applied(such as use of a confining member in combination with the solid-statebattery). In general, a failure occurs in the solid-state battery whenan unexpected local pressure is applied to the solid-state battery.Also, in the case of using the confining member in combination with thesolid-state battery, a predetermined pressure is generally applied tothe whole solid-state battery.

The confining member may be a member that can apply a confining pressureto the laminate of the two electrodes and the electrolyte layer disposedtherebetween, in the approximately parallel direction to the laminatingdirection. A known solid-state battery confining member may be used incombination with the solid-state battery of the disclosed embodiments.As the known solid-state battery confining member, examples include, butare not limited to, a confining member comprising a pair of plates thatare used to sandwich the solid-state battery, one or more rods that areused to connect the two plates, and a controller that is connected tothe rod(s) and used to control the confining pressure by use of a screwstructure, etc. In this case, the confining pressure applied to thesolid-state battery can be controlled by appropriately controlling thecontroller.

The confining pressure is not particularly limited. It may be 0.1 MPa ormore, may be 1 MPa or more, or may be 5 MPa or more. When the confiningpressure is 0.1 MPa or more, the layers constituting the solid-statebattery are in better contact with each other. On the other hand, theconfining pressure may be 100 MPa or less, may be 50 MPa or less, or maybe 20 MPa or less, for example. When the confining pressure is 100 MPaor less, it is not needed to use the special confining member.

EXAMPLE

Hereinafter, the disclosed embodiments will be further clarified by thefollowing examples. The disclosed embodiments are not limited to thefollowing examples, however.

1. Production of an Evaluation Sample

Example 1

(1) Production of a Laminate of a PTC Resistor Layer and an AluminumFoil

The following materials for a first slurry were prepared.

-   -   Carbon-containing electroconductive material: Furnace black        (manufactured by: Tokai Carbon Co., Ltd., average primary        particle diameter: 66 nm)    -   Insulating inorganic substance: Alumina (product name: CB-P02,        manufactured by: Showa Denko K. K., average particle diameter        (D₅₀): 2 μm)    -   Fluorine-containing polymer: PVDF (product name: KF POLYMER L        #9130, manufactured by: Kureha Corporation)    -   Non-aqueous solvent: N-methylpyrrolidone

The furnace black, the PVDF and the alumina were mixed at a volume ratioof 10:30:60 to prepare a mixture. The N-methylpyrrolidone was added tothe mixture, thereby producing the first slurry. Then, the first slurrywas applied on an aluminum foil having a thickness of 15 μm (a currentcollector). The applied first slurry was dried in a stationary dryingoven at 100° C. for one hour, thereby forming a first coating layerhaving a thickness of 9 μm.

The following materials for a second slurry were prepared.

-   -   Carbon-containing electroconductive material: Furnace black        (manufactured by: Tokai Carbon Co., Ltd., average primary        particle diameter: 66 nm)    -   Fluorine-containing polymer: PVDF (product name: KF POLYMER L        #9130, manufactured by: Kureha Corporation)    -   Non-aqueous solvent: N-methylpyrrolidone

First, the furnace black, the N-methylpyrrolidone and zirconia balls(diameter: 3 μm) were mixed and stirred by a ball mill (product name:AV-1, manufactured by: Asahi-Rika Seisakusho, K.K.) for 300 minutes toobtain a mixture. The PVDF was added to the mixture, and the mixture wasfurther stirred by the ball mill for 1200 minutes. At this time, theamount of the added PVDF was controlled so that the furnace black andthe PVDF were at a volume ratio of 40:60. The zirconia balls wereremoved from the mixture by classification, thereby preparing the secondslurry.

The second slurry was applied on the first coating layer of the laminateof the first coating layer and the aluminum foil by a doctor blademethod, thereby forming a second coating layer having a thickness of 3μm. After the second coating layer was formed, the second coating layerand the first coating layer were appropriately mixed and combined,thereby forming the PTC resistor layer.

The above step was carried out twice to produce two laminates of the PTCresistor layer and the aluminum foil (laminates A).

(2) Production of a Laminate of a Cathode Active Material Layer and anAluminum Foil

The following materials for the cathode active material layer were putin a container to obtain a mixture.

-   -   Cathode active material: LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ particles        (average particle diameter: 6 μm)    -   Sulfide-based solid electrolyte: Li₂S—P₂S₅-based glass ceramic        particles containing LiI and LiBr (average particle diameter:        0.8 μm)    -   Electroconductive material: VGCF    -   Binder: A 5 mass % solution of a PVDF-based binder in butyl        butyrate

The mixture in the container was stirred by an ultrasonic disperser(product name: UH-50, manufactured by: SMT Co., Ltd.) for 30 seconds.Next, the container was shaken by a shaking device (product name: TTM-1,manufactured by: Sibata Scientific Technology Ltd.) for three minutes.The mixture in the container was further stirred by the ultrasonicdisperser for 30 seconds, thereby preparing a slurry for forming thecathode active material layer.

Using an applicator, the slurry for forming the cathode active materiallayer was applied to one surface of an aluminum foil (serving as acathode current collector, manufactured by Showa Denko K. K.) by thedoctor blade method. The applied slurry was dried on a hot plate at 100°C. for 30 minutes, thereby forming the cathode active material layer onone surface of the aluminum foil.

The above step was carried out twice to produce two laminates of thecathode active material layer and the aluminum foil (laminates B).

(3) Production of an Evaluation Sample

First, a laminate C was produced by use of the two laminates B. Thelaminate C had the following layer structure: cathode active materiallayer/aluminum foil/cathode active material layer. Details are asfollows.

The two laminates B were laminated so that the cathode active materiallayer of one laminate B and the aluminum foil of the other laminate Bwere in contact with each other. A laminate thus obtained was subjectedto roll pressing at 10 kN/cm in a room temperature condition, therebyobtaining a laminate having the following layer structure: cathodeactive material layer/aluminum foil/cathode active materiallayer/aluminum foil. The aluminum foil disposed outside the laminate waspeeled off from the laminate. The laminate was subjected to rollpressing at 50 kN/cm at 165° C. to densify the two cathode activematerial layers, thereby obtaining a laminate having the following layerstructure: cathode active material layer/aluminum foil/cathode activematerial layer (the laminate C).

The laminate C was disposed between the laminates A and they werelaminated so that the cathode active material layers of the laminate Cwere in contact with the PTC resistor layers of the laminates A, therebyobtaining the evaluation sample of Example 1 having the following layerstructure: aluminum foil/PTC resistor layer/cathode active materiallayer/aluminum foil/cathode active material layer/PTC resistorlayer/aluminum foil.

A cross section of the evaluation sample of Example 1 was the same asthe evaluation sample 50 shown in FIG. 3. As shown in FIG. 3, the layerstructure of the evaluation sample 50 was as follows: current collector3 (aluminum foil)/PTC resistor layer 1/cathode active material layer2/current collector 3′ (aluminum foil)/cathode active material layer2/PTC resistor layer 1/current collector 3 (aluminum foil). As is clearfrom FIG. 3, the evaluation sample 50 was formed by disposing thecurrent collector 3′ (aluminum foil) between the two electrodes 10 forsolid-state batteries.

Example 2

The evaluation sample of Example 2 was produced in the same manner asExample 1, except for the following three respects in “(1) Production ofa laminate of a PTC resistor layer and an aluminum foil”.

-   -   The volume ratio of the furnace black to the PVDF in the second        slurry was changed from 40:60 to 20:80.    -   The second slurry applying method was changed from the doctor        blade method to a gravure coating method.    -   The thickness of the second coating layer formed by use of the        second slurry was changed from 3 μm to 2 μm.

Example 3

The evaluation sample of Example 3 was produced in the same manner asExample 2, except that after the laminates A were produced (see “(1)Production of a laminate of a PTC resistor layer and an aluminum foil”in Example 1), the laminates A were subjected to roll pressing under theconditions of a line pressure of 5.6 kN/cm and room temperature.

Example 4

The evaluation sample of Example 4 was produced in the same manner asExample 2, except that after the laminates A were produced (see “(1)Production of a laminate of a PTC resistor layer and an aluminum foil”in Example 1), the laminates A were subjected to roll pressing under theconditions of a line pressure of 14.2 kN/cm and room temperature.

Comparative Example 1

The evaluation sample of Comparative Example 1 was produced in the samemanner as Example 1, except for the following two respects.

-   -   In “(1) Production of a laminate of a PTC resistor layer and an        aluminum foil”, two laminates A′ were produced, each of which        was a laminate of the aluminum foil and the PTC resistor layer        that was composed of the first coating layer only (the second        coating layer was not formed).    -   In “(3) Production of an evaluation sample”, the laminate C was        disposed between the laminates A′ and they were laminated so        that the cathode active material layers of the laminate C were        in contact with the PTC resistor layers (each composed of the        first coating layer only) of the laminates A′, thereby producing        the evaluation sample of Comparative Example 1 having the        following layer structure: aluminum foil/(PTC resistor layer        composed of the first coating layer only)/cathode active        material layer/aluminum foil/cathode active material layer/(PTC        resistor layer composed of the first coating layer        only)/aluminum foil.

That is, the evaluation sample of Comparative Example 1 differs from theevaluation sample of Example 1 in the following respect: in place of thelaminates A, the evaluation sample of Comparative Example 1 included thelaminates A′, in each of which the second coating layer was not formed.

2. Evaluation of the Evaluation Samples

The evaluation samples of Examples 1 to 4 and Comparative Example 1 wereevaluated as follows. The results are shown in Table 1.

(1) Measurement of Hardness

For each of the laminates A (the laminates of the PTC resistor layer andthe aluminum foil) used to produce the evaluation samples, the hardnessof the PTC resistor layer-side surface was measured. The PTC resistorlayer-side surface serves as the electrode active materiallayer-contacting surface when it is incorporated in the evaluationsample.

The hardness was measured by use of a nanoindenter (product name: G200,manufactured by: MTS Systems Corporation) and in accordance with ISO14577:2015 (Part. 1: Test method). More specifically, 15 points at anindentation depth of from 400 nm to 600 nm were measured for hardness(GPa), and the average of the hardness values thus obtained wasdetermined as the hardness of each evaluation sample.

(2) Calculation of the C/F Value

First, each evaluation sample was subjected to cross section polishingby a cross section polisher (CP), thereby obtaining two cross sections.One of the cross sections of the PTC resistor layer was observed with afield emission-scanning electron microscope (FE-SEM), thereby obtaininga SEM image.

Next, on the SEM image, the PTC resistor layer was divided into the Alayer and the B layer in order from nearest to furthest from theelectrode active material layer so that, at any point of the PTCresistor layer, the ratio of the thickness of the A layer to thethickness of the B layer was 1:2 in the thickness direction of the PTCresistor layer. Accordingly, on the SEM image, the cross-sectional areaof the A layer was one-third of the cross-sectional area of the wholePTC resistor layer, and the cross-sectional area of the B layer wastwo-thirds of the cross-sectional area of the whole PTC resistor layer.

Next, a cross section of the A layer of each evaluation sample wasdefined by use of the SEM image and subjected to elemental analysis byflat quad-energy dispersive X-ray spectroscopy (FQ-EDX). From theresults of the elemental analysis, the amount (atomic %) of C containedin the cross section of the A layer and the amount (atomic %) of Fcontained therein were calculated. The C amount (atomic %) was dividedby the F amount (atomic %), and the resulting value was used as the C/Fvalue of the evaluation sample.

(3) Measurement of the Electronic Resistance

As shown in FIG. 3, a tester (“40” in FIG. 3, product name: RM3545,manufactured by: Hioki E.E. Corporation) was connected to the evaluationsample 50, thereby producing the circuit 200 for electronic resistancemeasurement. The electronic resistance of the evaluation sample 50 in aroom temperature (25° C.) condition and the electronic resistancethereof in a temperature condition of 250° C., were measured by use ofthe circuit 200 for electronic resistance measurement.

In Example 3, the electronic resistance measurement in the temperaturecondition of 250° C. was not carried out. The reason is as follows. Theevaluation sample of Example 3 was produced in the same manner as theevaluation samples of Examples 2 and 4, except for the roll pressingpressure condition. As shown in the following Table 1, the electronicresistance values of Examples 2 and 4 in the temperature condition of250° C. are high. Accordingly, it is clear that the electronicresistance value of the evaluation sample of Example 3 in thetemperature condition of 250° C. is also high. Accordingly, theelectronic resistance measurement in the same temperature condition wasnot carried out in Example 3.

The following Table 1 is a table comparing the production conditions andevaluation results of the evaluation samples of Examples 1 to 4 andComparative Example 1. Of the items relating to electronic resistanceshown in Table 1, “Room temperature (%)” shows the relative values (%)of the electronic resistances of Examples 1 to 4 in the room temperaturecondition when the electronic resistance value of Comparative Example 1in the room temperature condition is determined as 100%; “250° C. (%)”shows the relative values (%) of the electronic resistances of Examples1 to 4 in the temperature condition of 250° C. when the electronicresistance value of Comparative Example 1 in the temperature conditionof 250° C. is determined as 100%; and “250° C./Room temperature” showsthe value obtained by dividing the electronic resistance value (actualmeasured value) of each evaluation sample in the temperature conditionof 250° C. by the electronic resistance value (actual measured value)thereof in the room temperature condition. That is, the value of “250°C./Room temperature” is a value indicating how many times larger theelectronic resistance value in the temperature condition of 250° C. isthan the electronic resistance value in the room temperature condition.

TABLE 1 Production conditions Evaluation items Roll Electronicresistance Second coating layer pressing Room C/PVDF Thickness pressureHardness temperature 250° C. 250° C./Room (Volume ratio) (μm) (kN/cm)(GPa) C/F value (%) (%) temperature Example 1 40/60 3.0 — 0.36 3.1 17205 169 Example 2 20/80 2.0 — 0.23 2.4 13 6049 6563 Example 3 20/80 2.0 5.6 0.25 3.9 11 — — Example 4 20/80 2.0 14.2 0.22 2.6 10 14279 19991Comparative — — — 0.52 4.8 100 100 14 Example 13. Conclusion

According to Table 1, the hardness of Comparative Example 1 is 0.52 GPa.This result indicates that for the evaluation sample of ComparativeExample 1, the hardness of the electrode active materiallayer-contacting surface of the PTC resistor layer, is too hard. The C/Fvalue of Comparative Example 1 is 4.8. This result indicates that forthe evaluation sample of Comparative Example 1, the composition of apart of the PTC resistor layer, which is on the side near the electrodeactive material layer, contains too much carbon.

Meanwhile, the hardnesses of Examples 1 to 4 are 0.36 GPa or less. Theseresults indicate that for the evaluation samples of Examples 1 to 4, thehardness of the electrode active material layer-contacting surface ofthe PTC resistor layer, is moderately soft, and the PTC resistor layershows moderate contact and followability with respect to the electrodeactive material layer. The C/F values of Examples 1 to 4 are from 2.4 to3.9. These results indicate that for the evaluation samples of Examples1 to 4, the composition of the part of the PTC resistor layer, which ison the side near the electrode active material layer, keeps a goodbalance between the carbon content ratio and the fluorine content ratio.

According to Table 1, each of the electronic resistance values ofExamples 1 to 4 in the room temperature condition, is from 10% to 17% ofthe electronic resistance value of Comparative Example 1 in the roomtemperature condition.

Accordingly, the following was proved: the hardness of the electrodeactive material layer-contacting surface of the PTC resistor layer, isthe specific value or less, and the content ratio of carbon atoms withrespect to fluorine atoms in the A layer that is near the electrodeactive material layer, is in the specific value range, whereby theelectrode active material layer-contacting surface of the PTC resistorlayer has better flexibility than ever before, and excellent contactbetween the PTC resistor layer and the electrode active material layeris obtained, resulting in excellent followability of the PTC resistorlayer to the electrode active material layer surface. As a result, whenthe electrode is used in a solid-state battery, an increase in theelectronic resistance at the interface between the PTC resistor layerand the electrode active material layer, is suppressed, and a decreasein the performance of the solid-state battery is suppressed.

Also, the electronic resistance values of Examples 1, 2 and 4 in thetemperature condition of 250° C., are 169 times or higher the electronicresistance values thereof in the room temperature condition (see “250°C./Room temperature” in Table 1). It is presumed that the electronicresistance value of Example 3 in the temperature condition of 250° C. isalso higher than the electronic resistance value in the room temperaturecondition. This indicates that since the C/F values are from 2.4 to 3.9,the electrodes for solid-state batteries of Example 1 to 4 are excellentin thermal stability.

As described above, the results of the electronic resistance measurementof the evaluation samples can be said to be test results reflecting theperformance of the solid-state battery itself (FIG. 4).

REFERENCE SIGNS LIST

-   1. PTC resistor layer-   1 a. A layer-   1 b. B layer-   2. Electrode active material layer-   3, 3′. Current collector-   10. Electrode for solid-state batteries-   20. Electrolyte layer-   30. Opposite electrode-   40. Tester-   50. Evaluation sample-   100. Solid-state battery-   200. Circuit for electronic resistance measurement

The invention claimed is:
 1. An electrode for solid-state batteries, theelectrode comprising: an electrode active material layer; a currentcollector; and a PTC resistor layer disposed between the electrodeactive material layer and the current collector, the PTC resistor layerbeing in contact with the electrode active material layer, wherein thePTC resistor layer contains a carbon-containing electroconductivematerial, an insulating inorganic substance and a fluorine-containingpolymer, wherein a hardness of a surface of the PTC resistor layer thatcontacts the electrode active material layer, is 0.36 GPa or less,wherein the PTC resistor layer is divided into an A layer and a B layerin order from nearest to furthest from the electrode active materiallayer so that, at any point of the PTC resistor layer, a ratio of athickness of the A layer to a thickness of the B layer is 1:2 in athickness direction of the PTC resistor layer, wherein a value obtainedby dividing an atomic percentage of carbon atoms contained in the Alayer by an atomic percentage of fluorine atoms contained in the A layeris from 2.4 to 3.9, and wherein a content of the insulating inorganicsubstance in the A layer is smaller than a content of the insulatinginorganic substance in the B layer.
 2. The electrode for solid-statebatteries according to claim 1, wherein the hardness of the surface ofthe PTC resistor layer that contacts the electrode active material layeris from 0.22 GPa to 0.36 GPa.
 3. The electrode for solid-state batteriesaccording to claim 1, wherein the insulating inorganic substance is ametal oxide.
 4. The electrode for solid-state batteries according toclaim 1, wherein the carbon-containing electroconductive material iscarbon black.
 5. A solid-state battery comprising a cathode, an anodeand an electrolyte layer disposed between the cathode and the anode,wherein at least one of the cathode and the anode is the electrode forsolid-state batteries defined by claim
 1. 6. The electrode forsolid-state batteries according to claim 1, wherein a carbon amount inthe A layer is larger than a carbon amount in the B layer.